Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-09-27
2003-03-04
Nguyen, Patricia (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000, C330S138000
Reexamination Certificate
active
06529072
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a predistortion-based amplifier for compensating for distortion generated thereby when it amplifies a modulated wave signal based on envelope information of the modulated wave signal in accordance with a predistortion scheme, and more particularly, to a predistortion-based amplifier for improving an envelope information detecting efficiency and detecting accuracy.
2. Description of the Related Art
The predistortion scheme, which is known as a technique for linearizing amplifiers, detects an envelope using an envelope detector to compensate for distortions generated by an amplifier based on the result of the detection.
The predistortion scheme will now be described.
In a power amplifier used in a transmitter, its input/output characteristics include nonlinearity which causes distortions, resulting in unnecessary power radiation to the outside of a transmission frequency band. This causes interfering power to other frequency bands. For this reason, such a power amplifier is required to minimize the distortions generated thereby.
Also, since the power amplifier is a component which consumes large power, a reduction in power consumption of the power amplifier is regarded as an important issue because in a base station unit of a portable telephone system or the like, for example, a power amplifier consumes a majority of the power consumed by the base station unit.
However, for realizing a linear amplification using a power amplifier, it is necessary to operate the power amplifier in a region in which a direct current-to-alternating current converting efficiency (in this disclosure, hereinafter simply referred to as the “efficiency”) is low. In other words, an improvement on the nonlinearity of the power amplifier and a reduction in power consumption are in a mutually conflicting relationship, so that it is a difficult challenge to simultaneously solve the two problems.
As a method for simultaneously solving the two problems as mentioned, the predistortion scheme has been proposed.
The predistortion scheme is one approach to reduce distortions which applies, for example, distortions opposite to those generated by a power amplifier to a stage previous to an input of the power amplifier (distortion for canceling the distortions generated by the power amplifier) to improve the distortion characteristic of the power amplifier. In addition to this predistortion scheme, a scheme which applies such opposite distortions in a baseband is referred to as a baseband predistortion scheme; a scheme which applies such opposite distortions in an intermediate frequency (IF) band is referred to as an IF predistortion scheme; and a scheme which applies such opposite distortions in a radio frequency (RF) band is referred to as an RF predistortion scheme.
Generally, the nonlinearity found in transmission power amplifiers is divided into an amplitude component and a phase component. Then, the nonlinearity in the amplitude component is generally represented by how the gain changes with respect to input power supplied to a power amplifier, referred to as an AM (Amplitude Modulation)—AM characteristic. The nonlinearity in the phase component in turn is represented by how the phase rotates with respect to the input power supplied to the power amplifier, referred to as an AM-PM (Phase Modulation) characteristic.
The predistortion scheme as mentioned above uses, for example, a voltage controlled variable attenuator to generate distortions having an opposite characteristic to the AM-AM characteristic of a power amplifier at a stage previous to the power amplifier, and uses a voltage controlled variable phaser to generate distortions having an opposite characteristic to the AM-PM characteristic at a stage previous to the power amplifier, thereby reducing distortions in amplitude and phase generated by the power amplifier.
In the RF predistortion scheme and IF predistortion scheme, for example, a variable attenuator and a variable phase must be operated in accordance with the envelope of a high frequency signal which is to be amplified. Generally, with a transmission bandwidth of 20 MHz, by way of example, the envelope has a frequency component of 20 MHz at maximum. Therefore, such a predistortion scheme requires an envelope information detector circuit which has an input bandwidth as wide as the transmission bandwidth.
Exemplary configurations of circuits using the predistortion schemes as mentioned above are disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-69098 and Japanese Patent Application No. 2000-260214.
Next, the envelope information detector used in the aforementioned RF predistortion scheme and IF predistortion scheme will be described.
FIG. 9
illustrates an exemplary circuit configuration of an envelope information detector which may be included in a conventional predistortion-based amplifier. The illustrated envelope information detector comprises a matching capacitor
41
, a diode
42
, a matching coil
43
, an RF choke coil
44
, a biasing voltage source
45
, a capacitor
46
, and a resistor
47
. For the diode
42
, a Schottky diode, for example, is employed because of its ability to operate at a high speed and detect high frequencies.
Specifically, in the envelope information detector illustrated in
FIG. 9
, an input terminal D
1
which receives a modulated wave signal is connected to one end of the matching capacitor
41
, the other end of which is connected to an anode of the diode
42
. A cathode of the diode
42
is connected to an output terminal D
2
. A point between the input terminal D
1
and the matching capacitor
41
is grounded through the matching coil
43
, and the biasing voltage source
45
is connected between the matching capacitor
41
and the diode
42
through the RF choke coil
44
. Also, one end of the capacitor
46
and one end of the resistor
47
are connected in parallel between the diode
42
and the output terminal D
2
, and the other end of the capacitor
46
and the other end of the resistor
47
are grounded.
Also, in the envelope information detector illustrated in
FIG. 9
, the matching capacitor
41
and the matching coil
43
constitute a matching circuit, while the capacitor
46
and the resistor
47
constitute a low pass filter (LPF).
An exemplary operation of the envelope information detector illustrated in
FIG. 9
will be described below.
In the envelope information detector, a modulated wave signal input to the input terminal D
1
is applied to the anode of the diode
42
through the matching circuit. At the anode of the diode
42
, the input modulated wave signal is provided as a voltage signal. Also, for example, if the capacitor
46
has a sufficiently large capacitance with respect to the carrier frequency of the modulated wave signal so that it is regarded as being short-circuited at the carrier frequency in the AC domain, the modulated wave signal provided at the anode of the diode
42
is converted to a current signal by the diode
42
.
Simultaneously, the modulated wave signal is rectified by the ON-OFF characteristic of the diode
42
, whereby the modulated wave signal is separated into a carrier component and an envelope component. Then, the carrier component included in the converted current signal is removed by the LPF, so that the envelope component alone is detected by the resistor
47
as a voltage signal which is output from the output terminal D
2
.
With the operation as described above, the envelope information detector detects the envelope component of the input modulated wave signal.
As an example of conventional techniques, a modulated output detector circuit for a radio device described in Japanese Patent Application Laid-Open No. 5-251971 uses a quarter wavelength transmission path which functions as a diode or a filter for detecting a modulated output from a high power amplifier, resistor for leading out a detected output, and so on in an ALC (Automatic Level Control) circuit for controlling the transmission power to
Hitachi Kokusai Electric Inc.
Nguyen Patricia
Wenderoth , Lind & Ponack, L.L.P.
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